What sets up sys.path with Python, and when?

Question

When I run

import sys  print sys.path 

on my Mac (Mac OS X 10.6.5, Python 2.6.1), I get the following results.

 /Library/Python/2.6/site-packages/ply-3.3-py2.6.egg ... /Library/Python/2.6/site-packages/ipython-0.10.1-py2.6.egg /System/Library/Frameworks/Python.framework/Versions/2.6/lib/python26.zip /System/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6 /System/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/plat-darwin /System/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/plat-mac /System/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/plat-mac/lib-scriptpackages  /System/Library/Frameworks/Python.framework/Versions/2.6/Extras/lib/python /System/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/lib-tk /System/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/lib-old /System/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/lib-dynload /Library/Python/2.6/site-packages /System/Library/Frameworks/Python.framework/Versions/2.6/Extras/lib/python/PyObjC /System/Library/Frameworks/Python.framework/Versions/2.6/Extras/lib/python/wx-2.8-mac-unicode 

They are grouped into 5 categories.

• /Library/Python/2.6/site-packages/*.egg
• /Library/Python/2.6/site-packages
• Frameworks/Python.framework/Versions/2.6/lib/python2.6
• Frameworks/Python.framework/Versions/2.6/Extras/lib/python
• PATH from PYTHONPATH environment variable.

And I can add more paths using the code

sys.path.insert(0, MORE_PATH) 

• What routines sets up those paths, and when?
• Are some of the paths are built in python source code?
• Is it possible that the paths inserted with 'sys.path.insert' are ignored? I'm curious about this, as with mod_wsgi, I found the paths are not found with 'sys.path.insert'. I asked another post for this question.

ADDED

Based on Michael's answer, I looked into site.py, and I got the following code.

def addsitepackages(known_paths):     """Add site-packages (and possibly site-python) to sys.path"""     sitedirs = []     seen = []      for prefix in PREFIXES:         if not prefix or prefix in seen:             continue         seen.append(prefix)          if sys.platform in ('os2emx', 'riscos'):             sitedirs.append(os.path.join(prefix, "Lib", "site-packages"))         elif sys.platform == 'darwin' and prefix == sys.prefix:             sitedirs.append(os.path.join("/Library/Python", sys.version[:3], "site-packages")) 

I also think that the directory name that has site.py (/System/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6 for my Mac) should be built into Python source code.

Answer 1

Most of the stuff is set up in Python's site.py which is automatically imported when starting the interpreter (unless you start it with the -S option). Few paths are set up in the interpreter itself during initialization (you can find out which by starting python with -S).

Additionally, some frameworks (like Django I think) modify sys.path upon startup to meet their requirements.

The site module has a pretty good documentation, a commented source code and prints out some information if you run it via python -m site.